/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import static com.google.common.base.Preconditions.checkNotNull;
import static java.util.Objects.requireNonNull;

import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.annotations.VisibleForTesting;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import com.google.errorprone.annotations.DoNotCall;
import com.google.errorprone.annotations.concurrent.LazyInit;
import com.google.j2objc.annotations.WeakOuter;

import java.io.Serializable;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.function.Function;
import java.util.function.ToIntFunction;
import java.util.stream.Collector;
import javax.annotation.CheckForNull;

import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * A {@link Multiset} whose contents will never change, with many other important properties
 * detailed at {@link ImmutableCollection}.
 *
 * <p><b>Grouped iteration.</b> In all current implementations, duplicate elements always appear
 * consecutively when iterating. Elements iterate in order by the <i>first</i> appearance of that
 * element when the multiset was created.
 *
 * <p>See the Guava User Guide article on <a href=
 * "https://github.com/google/guava/wiki/ImmutableCollectionsExplained"> immutable collections</a>.
 *
 * @author Jared Levy
 * @author Louis Wasserman
 * @since 2.0
 */
@GwtCompatible(serializable = true, emulated = true)
@SuppressWarnings("serial") // we're overriding default serialization
@ElementTypesAreNonnullByDefault
public abstract class ImmutableMultiset<E> extends ImmutableMultisetGwtSerializationDependencies<E>
        implements Multiset<E>
{

    /**
     * Returns a {@code Collector} that accumulates the input elements into a new {@code
     * ImmutableMultiset}. Elements iterate in order by the <i>first</i> appearance of that element in
     * encounter order.
     *
     * @since 21.0
     */
    public static <E> Collector<E, ?, ImmutableMultiset<E>> toImmutableMultiset()
    {
        return CollectCollectors.toImmutableMultiset(Function.identity(), e -> 1);
    }

    /**
     * Returns a {@code Collector} that accumulates elements into an {@code ImmutableMultiset} whose
     * elements are the result of applying {@code elementFunction} to the inputs, with counts equal to
     * the result of applying {@code countFunction} to the inputs.
     *
     * <p>If the mapped elements contain duplicates (according to {@link Object#equals}), the first
     * occurrence in encounter order appears in the resulting multiset, with count equal to the sum of
     * the outputs of {@code countFunction.applyAsInt(t)} for each {@code t} mapped to that element.
     *
     * @since 22.0
     */
    public static <T extends @Nullable Object, E>
    Collector<T, ?, ImmutableMultiset<E>> toImmutableMultiset(
            Function<? super T, ? extends E> elementFunction,
            ToIntFunction<? super T> countFunction)
    {
        return CollectCollectors.toImmutableMultiset(elementFunction, countFunction);
    }

    /**
     * Returns the empty immutable multiset.
     *
     * <p><b>Performance note:</b> the instance returned is a singleton.
     */
    @SuppressWarnings("unchecked") // all supported methods are covariant
    public static <E> ImmutableMultiset<E> of()
    {
        return (ImmutableMultiset<E>) RegularImmutableMultiset.EMPTY;
    }

    /**
     * Returns an immutable multiset containing a single element.
     *
     * @throws NullPointerException if {@code element} is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E element)
    {
        return copyFromElements(element);
    }

    /**
     * Returns an immutable multiset containing the given elements, in order.
     *
     * @throws NullPointerException if any element is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E e1, E e2)
    {
        return copyFromElements(e1, e2);
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any element is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3)
    {
        return copyFromElements(e1, e2, e3);
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any element is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4)
    {
        return copyFromElements(e1, e2, e3, e4);
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any element is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4, E e5)
    {
        return copyFromElements(e1, e2, e3, e4, e5);
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any element is null
     * @since 6.0 (source-compatible since 2.0)
     */
    public static <E> ImmutableMultiset<E> of(E e1, E e2, E e3, E e4, E e5, E e6, E... others)
    {
        return new Builder<E>().add(e1).add(e2).add(e3).add(e4).add(e5).add(e6).add(others).build();
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any of {@code elements} is null
     * @since 6.0
     */
    public static <E> ImmutableMultiset<E> copyOf(E[] elements)
    {
        return copyFromElements(elements);
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableMultiset<E> copyOf(Iterable<? extends E> elements)
    {
        if (elements instanceof ImmutableMultiset)
        {
            @SuppressWarnings("unchecked") // all supported methods are covariant
            ImmutableMultiset<E> result = (ImmutableMultiset<E>) elements;
            if (!result.isPartialView())
            {
                return result;
            }
        }

        Multiset<? extends E> multiset =
                (elements instanceof Multiset)
                        ? Multisets.cast(elements)
                        : LinkedHashMultiset.create(elements);

        return copyFromEntries(multiset.entrySet());
    }

    /**
     * Returns an immutable multiset containing the given elements, in the "grouped iteration order"
     * described in the class documentation.
     *
     * @throws NullPointerException if any of {@code elements} is null
     */
    public static <E> ImmutableMultiset<E> copyOf(Iterator<? extends E> elements)
    {
        Multiset<E> multiset = LinkedHashMultiset.create();
        Iterators.addAll(multiset, elements);
        return copyFromEntries(multiset.entrySet());
    }

    private static <E> ImmutableMultiset<E> copyFromElements(E... elements)
    {
        Multiset<E> multiset = LinkedHashMultiset.create();
        Collections.addAll(multiset, elements);
        return copyFromEntries(multiset.entrySet());
    }

    static <E> ImmutableMultiset<E> copyFromEntries(
            Collection<? extends Entry<? extends E>> entries)
    {
        if (entries.isEmpty())
        {
            return of();
        }
        else
        {
            return RegularImmutableMultiset.create(entries);
        }
    }

    ImmutableMultiset()
    {
    }

    @Override
    public UnmodifiableIterator<E> iterator()
    {
        final Iterator<Entry<E>> entryIterator = entrySet().iterator();
        return new UnmodifiableIterator<E>()
        {
            int remaining;
            @CheckForNull
            E element;

            @Override
            public boolean hasNext()
            {
                return (remaining > 0) || entryIterator.hasNext();
            }

            @Override
            public E next()
            {
                if (remaining <= 0)
                {
                    Entry<E> entry = entryIterator.next();
                    element = entry.getElement();
                    remaining = entry.getCount();
                }
                remaining--;
                /*
                 * requireNonNull is safe because `remaining` starts at 0, forcing us to initialize
                 * `element` above. After that, we never clear it.
                 */
                return requireNonNull(element);
            }
        };
    }

    @LazyInit
    @CheckForNull
    private transient ImmutableList<E> asList;

    @Override
    public ImmutableList<E> asList()
    {
        ImmutableList<E> result = asList;
        return (result == null) ? asList = super.asList() : result;
    }

    @Override
    public boolean contains(@CheckForNull Object object)
    {
        return count(object) > 0;
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    public final int add(E element, int occurrences)
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    public final int remove(@CheckForNull Object element, int occurrences)
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    public final int setCount(E element, int count)
    {
        throw new UnsupportedOperationException();
    }

    /**
     * Guaranteed to throw an exception and leave the collection unmodified.
     *
     * @throws UnsupportedOperationException always
     * @deprecated Unsupported operation.
     */
    @CanIgnoreReturnValue
    @Deprecated
    @Override
    @DoNotCall("Always throws UnsupportedOperationException")
    public final boolean setCount(E element, int oldCount, int newCount)
    {
        throw new UnsupportedOperationException();
    }

    @GwtIncompatible // not present in emulated superclass
    @Override
    int copyIntoArray(Object[] dst, int offset)
    {
        for (Multiset.Entry<E> entry : entrySet())
        {
            Arrays.fill(dst, offset, offset + entry.getCount(), entry.getElement());
            offset += entry.getCount();
        }
        return offset;
    }

    @Override
    public boolean equals(@CheckForNull Object object)
    {
        return Multisets.equalsImpl(this, object);
    }

    @Override
    public int hashCode()
    {
        return Sets.hashCodeImpl(entrySet());
    }

    @Override
    public String toString()
    {
        return entrySet().toString();
    }

    /**
     * @since 21.0 (present with return type {@code Set} since 2.0)
     */
    @Override
    public abstract ImmutableSet<E> elementSet();

    @LazyInit
    @CheckForNull
    private transient ImmutableSet<Entry<E>> entrySet;

    @Override
    public ImmutableSet<Entry<E>> entrySet()
    {
        ImmutableSet<Entry<E>> es = entrySet;
        return (es == null) ? (entrySet = createEntrySet()) : es;
    }

    private ImmutableSet<Entry<E>> createEntrySet()
    {
        return isEmpty() ? ImmutableSet.<Entry<E>>of() : new EntrySet();
    }

    abstract Entry<E> getEntry(int index);

    @WeakOuter
    private final class EntrySet extends IndexedImmutableSet<Entry<E>>
    {
        @Override
        boolean isPartialView()
        {
            return ImmutableMultiset.this.isPartialView();
        }

        @Override
        Entry<E> get(int index)
        {
            return getEntry(index);
        }

        @Override
        public int size()
        {
            return elementSet().size();
        }

        @Override
        public boolean contains(@CheckForNull Object o)
        {
            if (o instanceof Entry)
            {
                Entry<?> entry = (Entry<?>) o;
                if (entry.getCount() <= 0)
                {
                    return false;
                }
                int count = count(entry.getElement());
                return count == entry.getCount();
            }
            return false;
        }

        @Override
        public int hashCode()
        {
            return ImmutableMultiset.this.hashCode();
        }

        @GwtIncompatible
        @Override
        Object writeReplace()
        {
            return new EntrySetSerializedForm<E>(ImmutableMultiset.this);
        }

        private static final long serialVersionUID = 0;
    }

    @GwtIncompatible
    static class EntrySetSerializedForm<E> implements Serializable
    {
        final ImmutableMultiset<E> multiset;

        EntrySetSerializedForm(ImmutableMultiset<E> multiset)
        {
            this.multiset = multiset;
        }

        Object readResolve()
        {
            return multiset.entrySet();
        }
    }

    @GwtIncompatible
    @Override
    Object writeReplace()
    {
        return new SerializedForm(this);
    }

    /**
     * Returns a new builder. The generated builder is equivalent to the builder created by the {@link
     * Builder} constructor.
     */
    public static <E> Builder<E> builder()
    {
        return new Builder<E>();
    }

    /**
     * A builder for creating immutable multiset instances, especially {@code public static final}
     * multisets ("constant multisets"). Example:
     *
     * <pre>{@code
     * public static final ImmutableMultiset<Bean> BEANS =
     *     new ImmutableMultiset.Builder<Bean>()
     *         .addCopies(Bean.COCOA, 4)
     *         .addCopies(Bean.GARDEN, 6)
     *         .addCopies(Bean.RED, 8)
     *         .addCopies(Bean.BLACK_EYED, 10)
     *         .build();
     * }</pre>
     *
     * <p>Builder instances can be reused; it is safe to call {@link #build} multiple times to build
     * multiple multisets in series.
     *
     * @since 2.0
     */
    public static class Builder<E> extends ImmutableCollection.Builder<E>
    {
        final Multiset<E> contents;

        /**
         * Creates a new builder. The returned builder is equivalent to the builder generated by {@link
         * ImmutableMultiset#builder}.
         */
        public Builder()
        {
            this(LinkedHashMultiset.<E>create());
        }

        Builder(Multiset<E> contents)
        {
            this.contents = contents;
        }

        /**
         * Adds {@code element} to the {@code ImmutableMultiset}.
         *
         * @param element the element to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code element} is null
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E element)
        {
            contents.add(checkNotNull(element));
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
         *
         * @param elements the elements to add
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> add(E... elements)
        {
            super.add(elements);
            return this;
        }

        /**
         * Adds a number of occurrences of an element to this {@code ImmutableMultiset}.
         *
         * @param element     the element to add
         * @param occurrences the number of occurrences of the element to add. May be zero, in which
         *                    case no change will be made.
         * @return this {@code Builder} object
         * @throws NullPointerException     if {@code element} is null
         * @throws IllegalArgumentException if {@code occurrences} is negative, or if this operation
         *                                  would result in more than {@link Integer#MAX_VALUE} occurrences of the element
         */
        @CanIgnoreReturnValue
        public Builder<E> addCopies(E element, int occurrences)
        {
            contents.add(checkNotNull(element), occurrences);
            return this;
        }

        /**
         * Adds or removes the necessary occurrences of an element such that the element attains the
         * desired count.
         *
         * @param element the element to add or remove occurrences of
         * @param count   the desired count of the element in this multiset
         * @return this {@code Builder} object
         * @throws NullPointerException     if {@code element} is null
         * @throws IllegalArgumentException if {@code count} is negative
         */
        @CanIgnoreReturnValue
        public Builder<E> setCount(E element, int count)
        {
            contents.setCount(checkNotNull(element), count);
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
         *
         * @param elements the {@code Iterable} to add to the {@code ImmutableMultiset}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterable<? extends E> elements)
        {
            if (elements instanceof Multiset)
            {
                Multiset<? extends E> multiset = Multisets.cast(elements);
                multiset.forEachEntry((e, n) -> contents.add(checkNotNull(e), n));
            }
            else
            {
                super.addAll(elements);
            }
            return this;
        }

        /**
         * Adds each element of {@code elements} to the {@code ImmutableMultiset}.
         *
         * @param elements the elements to add to the {@code ImmutableMultiset}
         * @return this {@code Builder} object
         * @throws NullPointerException if {@code elements} is null or contains a null element
         */
        @CanIgnoreReturnValue
        @Override
        public Builder<E> addAll(Iterator<? extends E> elements)
        {
            super.addAll(elements);
            return this;
        }

        /**
         * Returns a newly-created {@code ImmutableMultiset} based on the contents of the {@code
         * Builder}.
         */
        @Override
        public ImmutableMultiset<E> build()
        {
            return copyOf(contents);
        }

        @VisibleForTesting
        ImmutableMultiset<E> buildJdkBacked()
        {
            if (contents.isEmpty())
            {
                return of();
            }
            return JdkBackedImmutableMultiset.create(contents.entrySet());
        }
    }

    static final class ElementSet<E> extends ImmutableSet.Indexed<E>
    {
        private final List<Entry<E>> entries;
        // TODO(cpovirk): @Weak?
        private final Multiset<E> delegate;

        ElementSet(List<Entry<E>> entries, Multiset<E> delegate)
        {
            this.entries = entries;
            this.delegate = delegate;
        }

        @Override
        E get(int index)
        {
            return entries.get(index).getElement();
        }

        @Override
        public boolean contains(@CheckForNull Object object)
        {
            return delegate.contains(object);
        }

        @Override
        boolean isPartialView()
        {
            return true;
        }

        @Override
        public int size()
        {
            return entries.size();
        }
    }

    static final class SerializedForm implements Serializable
    {
        final Object[] elements;
        final int[] counts;

        // "extends Object" works around https://github.com/typetools/checker-framework/issues/3013
        SerializedForm(Multiset<? extends Object> multiset)
        {
            int distinct = multiset.entrySet().size();
            elements = new Object[distinct];
            counts = new int[distinct];
            int i = 0;
            for (Entry<? extends Object> entry : multiset.entrySet())
            {
                elements[i] = entry.getElement();
                counts[i] = entry.getCount();
                i++;
            }
        }

        Object readResolve()
        {
            LinkedHashMultiset<Object> multiset = LinkedHashMultiset.create(elements.length);
            for (int i = 0; i < elements.length; i++)
            {
                multiset.add(elements[i], counts[i]);
            }
            return ImmutableMultiset.copyOf(multiset);
        }

        private static final long serialVersionUID = 0;
    }
}
